1. Field of the Invention:
This invention concerns a method and a device for measuring the thickness of transparent materials. More especially, but not exclusively, the invention is aimed at measuring the thickness of materials of glass and, more specifically, measuring the thickness of glass containers, such as bottles or flasks.
2. Discussion of the Background:
The general quality requirements demanded by clients and the reduction in weight of bottles require a very rigorous control of the thickness of these bottles.
In the glassmaking industry it is usual, for obtaining thickness measurements, to use the measurement of electrical capacitance as a base. In effect, since glass is a dielectric material, measurement of the capacitance of the glass by means of electrodes enables the thickness of the glass to be locally determined.
Such a technique is described, notably, in Patents EP 300 616 and EP 363 114. It consists of applying a pick-up, in the form of an elongate electrode, onto a bottle while the bottle revolves about its axis.
Such a technique has the disadvantage of requiring the pick-up to be held against the wall of the bottle. This inevitably leads to rapid wear of the pick-up, amplified by the rotation of the bottle. The pick-up may also be entirely destroyed if the bottle has a large defect which strikes the pick-up.
Another technique is described in the French Patent published under number 2 129 416. This technique consists of establishing a high-frequency electro-magnetic field and measuring the variation in intensity of this field caused by the introduction of a dielectric material. The voltage induced in the probe gives an indication of the variation in intensity of the field and is proportional to the thickness of the dielectric material.
This technique does not, however, lead to high accuracy in measurements of thicknesses. It is more especially used for determining thicknesses below a threshold value without, however, giving an exact measurement.
Furthermore, it also requires contact with the bottle, which can result in premature wear of the measuring head, as has been described for the preceding technique.
This premature wear or accidental destructions therefore require a frequent replacement of the detectors or measuring heads and lead to high costs of maintenance of the control system.
Moreover, each deterioration and/or replacement of a pick-up or measuring head may necessitate a rechecking of a series of articles which has been inadequately checked and an interruption to production during the pause in the control, which therefore results in reduced output.